This invention relates to high strength, low alloy steels, particularly of low carbon category, having yield strength of at least about 80 ksi (80 thousand pounds per square inch), and very preferably superior toughness, especially at low temperatures. More particularly, the invention is concerned with steel products achieved by hot deformation, e.g. by hot rolling to desired thicknesses and shapes, including sheet, strip or the like, wherein the desired properties of yield strength, toughness and also advantageously ductility, fatigue resistance and weldability are attained with an economy of alloying ingredients.
Considerable demand exists for hot rolled steel products of the sort described above, which can be formed readily in a variety of fabricating operations such as in the automotive and other industries. Steels designed to serve these purposes, and having yield strength of the value noted above, have been proposed or made available, but to the extent that they have avoided an expensive content of alloying elements and have not required heat treatments that add to the cost or complexity of processing, such 80 ksi or like steels appear to have been somewhat deficient in respect to properties such as toughness and ductility.
Accordingly, an important aim of the present invention is to provide a new steel product which is of low alloy type and which not only has the basic properties of strength described above, but is characterized by improved ductility and greater toughness, notably improved impact strength at temperatures well below 0.degree. F, such improvements being considered in relation to prior steel products which might be considered approximately comparable in some other respects. Very good formability and exceptional toughness at very low temperatures are special objects of the invention, for correspondingly greater utility as to kinds of articles that may be fabricated from the steel, and greater latitude in their circumstances of use.